This is a very deep and complex historical question that causes many disputes. The simplified answer in the style of "Bulgarians are ungrateful" is not only incorrect but also hinders understanding the logic of international relations. The situation where Bulgaria found itself in the camp of the opponents of Russia in both world wars was the result of a tragic convergence of geopolitical interests, not the consequence of any innate ingratitude. To understand this, it is necessary to analyze Bulgaria's motivations separately for each war. Why did Bulgaria join World War I on the side of Germany and Austria-Hungary? The key reason was the Machiavellian principle of "the enemy of my enemy is my friend" and unfulfilled national interests. Geopolitical ambitions and "national unification". After being freed from the Ottoman yoke in 1878 (due to Russia), Bulgaria had the dream of uniting all lands with an ethnic Bulgarian population into one state (Great Bulgaria). The main enemy, holding these territories, was not Germany, but neighboring countries: Serbia held Macedonia (in the opinion of Bulgarians). Greece also claimed part of Macedonia. Romania held Southern Dobruja. The Second Balkan War (1913) — the root of the problem. This key event predetermined Bulgaria's choice in WWI. Dissatisfied with the division of the spoils after the First Balkan War, Bulgaria attacked its former allies — Serbia and Greece. The result was catastrophic: Bulgaria lost a large part of the conquered territories and even had to cede Southern Dobruja to Romania. Russia, the traditional patron of all Slavic peoples, tried to maintain neutrality in this conflict, but ultimately did not support Bulgaria because it could not afford to lose an ally in the form of Serbia. The choice of side in WWI. By 1915, when Bulgaria was deciding which side to join, it faced a choice: The Entente (Russia, France, Britain): offered to return only part of Macedonia but required Bulgaria to cede it ... Read more